From what I'm getting of it, there was some corroding of the skin layer of the comms ariel on the top of the plane. If that cracks and let's loose, there go pretty much all comms and also cabin decompression, passengers and pilots pass out, hence no calls, messages etc. it hits water and goes down deep. Black box and other transponders on this model aren't the best on the market, quite far from it, so if the radar/ariel has destroyed while pinning off the fuselage, you then have very little emissions coming from an older plane at a crazy depth, parts of the Indian Ocean have shown to be so deep as to be unable to gauge the depth.
One of many theories I'm sure. Just hope they find live people if I'm wrong!!